1. Field of the Invention
This invention relates to a method of brazing columbium to to itself and, more specifically, to a method of performing such brazing under low pressure and low temperature conditions while eventually producing a high remelt temperature.
2. Brief Description of the Prior Art
Columbium (Cb) is a desirable material for use in environments requiring high temperature regimes (i.e., about 2000.degree. to about 2500.degree. F.), such as for use in thermal protection systems. For these purposes, it is necessary that thin gauge columbium be bondable to itself below 2400.degree. F., the usual maximum temperature of vacuum brazing furnaces, and achieve a remelt temperature in excess of 2500.degree. F.
Brazing of columbium usually employs braze alloys which flow between 2600.degree. and 3100.degree. F. These braze alloys or fillers have high melting points to accommodate useful operations to 2500.degree. F. without remelt. If this high remelt temperature is not required, then columbium may be brazed at 1920.degree. F. using 48Ti-48Zr-4Be or 75Zr-19Cb-6Be. Columbium has been bonded at temperatures down to 1450.degree. F. using diffusion bonding techniques (with or without titanium interlayers), however, this technique requires significant bonding pressures (100 psi to 10 ksi) and, as such, does not lend itself to the bonding of intricately stiffened, thin gauge columbium sheets and the like.
In accordance with the prior art, if a high remelt temperature is required, then columbium may be brazed/bonded to itself using either high temperatures, about 2600.degree. F. and higher, with low pressure or alternatively, using low temperatures, about 1500.degree. to 2000.degree. F., with high pressure, in the above noted pressure range. If a low remelt temperature is permissible, then columbium may be brazed at 1920.degree. F.
Other noted prior art of interest is U.S. Pat. No. 3,046,650 which relates to braze bonding of columbium at high brazing temperatures (3138.degree. to 3182.degree. F.) and U.S. Pat. No. 4,331,286 appears to be generally concerned with aluminum and copper and uses an interlayer which will produce a low melting point eutectic with the base metals, the temperature being raised to the eutectic temperature to produce the eutectic liquid. From 2 to 10 ksi bonding pressure is then applied to squeeze out most of the eutectic liquid and cooling then takes place at a fast rate (10.degree. to 500.degree. C./sec) to prevent intermetallic phase formation and/or further liquid production.